1. Field of the Invention
The present invention relates generally to improvement in verifying reliability of an integrated circuit against electromigration, and more specifically to a tool of verifying design specifications of a CMOS (complementary metal oxide semiconductor) in connection with a peak current limit of the electromigration specifications.
2. Description of the Related Art
As is well known in the art, metal conductors in an integrated circuit have an upper current density limit imposed by electromigration, the movement of atoms from one place to another under the influence of electrical force. This effect limits the maximum current density of an aluminum (for example) conductor. More specifically, when a high current flows through thin metal conductors in an integrated circuit, metal ions in some regions will pile up and voids will form in other regions. The piled-up metal ions can short-circuit adjacent conductors while the voids can cause an open circuit.
A technique of verifying circuit reliability against electromigration is used to check portions of an integrated circuit, which may be affected by the electromigration, on the basis of data or information of integrated circuit designs.
One example of a conventional technique for investigating the electromigration is disclosed in U.S. Pat. No. 5,410,490 to Yastrow. This conventional technique examines the shapes of metal on a basis of an average of currents flowing through a variety of portions in the circuit. More specifically, data on a capacitance, an average frequency, and a voltage swing of a net are gathered, after which the average current needed to charge and discharge the net is computed. The average current is able to specify a minimum width of metal. Therefore, which do not satisfy the minimum width are checked and the result is displayed on the artwork.
According to the aforesaid conventional technique, the occurrences of electromigration are expected merely relying on the average current density. In other words, no consideration is given to an instantaneously occurring high current density (viz., a large amplitude pulse) by which the electromigration is abruptly induced. As metal conductors become very fine, it is very significant to avoid such an abrupt occurrence of electromigration. A study of a current pulse large enough to cause a metal line to fail instantaneously is disclosed in a paper titled "Short-Time Failure of Metal Interconnection Caused by Current Pulses" by James E. Murguia, et al., IEEE Electron Device Letters, Vol. 14, No 10, October 1993. In order to prevent the instantaneously occurring electromigration due to short-lived, high current pluses, it is necessary to design integrated circuits in consideration of maximum possible currents flowing through the circuits. That is, in the leading semiconductor fabrication field, it is vital to consider the peak current density in addition to the average currents.